Basements are rarely the most glamorous rooms in the house, but thanks to a combination of building restrictions and the super-rich, they’re growing in size and complexity in some cities.

In London they’re known as super-basements or iceberg basements. Wealthy residents who want palatial homes but planning laws and pricey land won’t allow them to expand up and out instead dig in. And it’s not just that these underground spaces span the width and depth of the houses. In some cases, they have several stories below the surface.

Canadian TV star David Graham made news several years ago with his plans to build a four-story house — three of which were underground. Up top is the kitchen, library, guest bedroom, and conservatory, while below is the three-car garage (with adjoining car elevator), swimming pool, and playroom. Some areas of the city have banned “excessive” below-ground extensions over noise concerns (it takes a lot to haul tons and tons of earth out of the ground) and fears that the basements could make adjacent properties unstable.

Another area with extremely expensive property and lots of millionaires is also seeing an increase in bigger basements: Silicon Valley. “What you’ll commonly see are spas, movie theaters, hobby rooms — you name it,” real estate agent Arthur Sharif told Mercury News. Other agents have seen subterranean spaces outfitted with pools, bowling alleys, racquetball or basketball courts, or cavernous garages for luxurious cars.

But who knows? One day it might be more than billionaires and their over-the-top underground lairs that are burrowing into the earth. Some futurists predict earthscrapers — multilevel buildings built underground instead of in the sky — could catch on as the world’s population grows and we run out of space to put everyone.